CHEM 301
Assignment #3
Provide solutions to the following questions in a neat and organized manner.
Clearly state assumptions and reference sources for any constants used.
Due date: November 15th.
Only even numbered questions will be assessed.
1. The diagram below depicts the dominant chemical speciation for Cu(II) as a function
of log [HCO3-] (M) and pH. Note: ‘azurite’ = Cu3(CO3)2(OH)2 (s) and ‘malachite’ =
Cu2CO3(OH)2 (s).
a) Predict the dominant form of Cu(II) in seawater.
b) Explain why the Cu2+/azurite boundary depends on the copper ion concentration.
c) Derive an expression for the log [HCO3-] as a function of pH in the form of
log [HCO3-] = m pH + b
2. A water sample has calcium hardness of 65.0 ppm CaCO3 and a total alkalinity of
120. ppm CaCO3. Calculate the pH at which the sample is saturated with respect to
calcite (Ksp(calcite) = 3.36 x 10-9).
3. The effect of increasing atmospheric concentrations on the aqueous solubility of
calcite and aragonite forms of calcium carbonate have significant environmental
consequence. The saturation index is defined as the ratio of the actual solubility product
activities over those at equilibrium as follows.
 = Qsp/Ksp
a) What does it mean if  < 1?
b) Which mineral form calcite or aragonite is more susceptible to aqueous dissolution
and why?
c) Use the formulas derived in class for the CO2-CaCO3-H2O system (or the PHREEQC
program described on pg261 of your textbook) to calculate the solubility of CaCO3 at
pre-industrial CO2 levels of 280 ppmv versus present day levels of 400 ppmv.
4. Methyl-t-butyl ether (MTBE) is a common gasoline additive used to improve the
combustion characteristics. Careless use over the years has resulted in groundwater
contamination. Using a reported value for KH of MTBE of 5.8 x 10-4 atm m3 mol-1,
calculate the partial pressure of MTBE in adjacent air-filled pores in the soil if the
groundwater contains 175 ppm of MTBE.
5. Using the pKa values below for the aluminum aquo complexes, plot a pH speciation
diagram for the Al3+ - Al(OH)2+ - Al(OH)2+ - Al(OH)3 – Al(OH)4- system over the pH
range of 0 to 14, using Excel for repetitive calculations. Calculate the fractional
abundance of aluminum species at pH 5?
pKa1 = 5.00
pKa2 = 5.10 pKa3 = 6.80 pKa4 = 5.80
6. A pollutant is dumped into a clean lake at a constant rate starting on July 1, 2000.
When the pollutant’s concentration reaches 90% of its steady state value, the flow of
pollutant is stopped. On what date will the concentration of pollutant fall to 1% of its
maximum concentration? Assume the rate constants of the increase and decrease are
both 0.35 year-1.
7. Consider dissolved organic matter to have a generic formula {CH2O}. For a water
body containing 1.0 mg/L of DOC, calculate the mass in mg of dissolved oxygen in the
same volume of water required to oxidize it completely. Use this calculation to establish
a numerical relationship between COD and DOC. Will your result change markedly
using the generic formula for dissolved humic material (Fig 12.3, textbook)?
8. Given the equilbria below, what fraction of lead species is present as Pb2+ at pH 7.
Pb2+ + OH- ====== PbOH+
K = 2.0 x 106
+
PbOH
+ OH ====== Pb(OH)2
K = 4.0 x 104
9. What fraction of NTA is uncomplexed after H2T- has been brought to equilibrium
with solid PbCO3 at pH 7 in aqueous solution with [HCO3-] = 1.25 x 10-3 M.
PbCO3(s) + HT2- ====== PbT- + HCO3log Ksp (PbCO3) = -13.13
log Kf (PbT-) = 11.34
10. A woodwaste leachate sample has a Fe(II) concentration of 20 mg/L and a dissolved
organic matter concentration of 50 mg/L. Using a typical value for the concentration of
carboxylate ions of per gram of DOM given in the textbook and the conditional formation
constant between Fe2+ and fulvic acid of 5 x 103 at pH 5, calculate the fraction of iron (II)
complexed to the dissolved organic matter at pH 5. How would you expect your answer
to change if the actual pH of the leachate were 7 rather than 5? Explain.